Kelleher will come head-to-head with the Leinstermen as his side Toulouse welcome Brian O'Driscoll and co to Le Stadium for a mouth-watering last-four tie on May 1.
Reigning European Champions and current Magners League leaders Leinster qualified for the semis thanks to a nail-biting win over French Championship runners up Clermont Auvergne on Friday night.
Two days later, Kelleher and his Toulouse colleagues became the fourth and final side to make it through thanks to a convincing home win over Stade Francias, conquerors of both Ulster and Edinburgh in the group stages.
"I watched Leinster play on Friday night and they are as good as anyone else," said Kelleher.
"We will not focus too much on them just yet but we know we have a few things to work on.
"Leinster are a difficult team. Seeing the level of rugby this weekend means our performance against Stade will not be sufficient to win the next game."
The semi-final battle between the three times European champs and the current holders will feature what could be a stunning midfield clash between two of the world's greatest centres.
Toulouse veteran Yannick Jauzion will line up against Leinster's own sensational centre O'Driscoll in what should be a hugely entertaining and delicately balanced match up.
"Brian is a world-class player and his performance against Clermont was exceptional," added Kelleher.
"We have Yannick and he was fantastic for us against Stade. If you look across Europe you won't see two better centres.
"But that's what semi-final rugby is all about. The knock-out stages are all about taking your chances because they don't come around too often. You may only get one or two opportunities.
"Quarter-finals and semi-finals are when your playmakers come into their own and players who can make the right decision become highly important."
As for Kelleher himself, the 33-year-old scrum-half admits he is hoping to gather some inside information on the Dubliners, although the broad smile on his face suggested he already knows the response he'll get when he tries to catch up with old Waikato team-mate and now Leinster coach Jono Gibbes.
"Jono is a coach there and he is a good, old Chiefs friend of mine. We will speak this week and try to catch up for a coffee and I'll try and ask for Leinster's gameplan!"
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